ASK THE INSPECTOR: No shortcuts when removing skylights

QUESTION: We have two skylights that were installed when we built our home in 1986. We have been having water leakage for the past five years or so and are considering totally removing them. Can you give us some guidance on what is involved? Would the attic skylight framing need to be totally removed for adequate attic air circulation? Your advice would be appreciated. Pat and Lionel Bernier

ANSWER: Removing troublesome skylights is an effective way to prevent recurrence of chronic leakage, but it must be done properly to work. Air sealing of the ceiling where the skylight opening currently is located is critical to prevent condensation and further moisture issues. Removal of the existing structure will be the easiest way to achieve this goal.

Skylights are often problematic simply because they create a thermal bridge between two separate components of the structure of your home -- the roof and ceiling. Homeowners often mix up these two items but, unless you have a home with a flat roof or an older vaulted ceiling, these are two very separate and distinct entities.

The roof is the top of the house, comprised of the framing, sheathing, and roof covering adjacent to the exterior of the house. The ceiling is the bottom of the roof framing, including the interior drywall or plaster finish and comprises the "floor" of the attic.

Residential skylights normally are installed on top of a short curb that is attached to the top cord of the rafters or trusses extending slightly above the finished roofing. The inside of the skylight is a box constructed of plywood or drywall sheathing which is attached to the inside of the curb at the top and the ceiling joists at the bottom. This is where the problems begin.

Because the skylight housing creates a bridge between the warm underside of the ceiling and the cold roof and attic, condensation is likely. This may occur on the underside of the skylight glazing itself, or on the inside of the skylight box. This box can sweat if it is not properly insulated or air-sealed, which will cause moisture damage and sometimes dripping into the home.

This is likely what you have experienced, unless the moisture was due to leakage from a poorly sealed curb at the exterior, which is equally possible. Heat escaping from the inside or around the skylight can also cause melting snow and ice to leak inside.

Whichever is the cause of your current moisture problem, removal of the complete skylight and housing will eliminate that possibility. If that is done, determining the specific culprit will not be necessary.

While most skylights are installed with several different components that make up the housing and the glazing, they essentially become a single system after complete. Removal of any of the parts of this system will usually require complete removal of the entire housing to allow proper repairs.

Once the entire unit is removed, it will expose the bare trusses or rafters, which will allow installation of additional framing to support the sheathing that will cover the old opening in the roof and ceiling. This is important to prevent sagging in the roofing or ceiling after the holes are filled in. It will also allow installation of a proper air/vapour barrier before the ceiling drywall to prevent air leakage into the old opening.

Once this new framing and polyethylene is in place, the ceiling and roof can be patched with drywall and plywood. After that, topping up the attic insulation and installing proper roofing will be possible. The final step will be taping and painting the new drywall to match the existing ceiling.

While it may be possible to complete this process without removing the existing skylight box, it will be considerably more difficult to properly seal and insulate this opening, which may lead to further moisture problems. Also, removal of the entire structure can only increase the air flow though the attic vents, which may have been partially blocked by the old skylight housing. This should improve the overall function of the attic ventilation system and further reduce the possibility of leakage from condensation.

When my clients ask about possible problems or leakage from skylights during pre-purchase inspections, I normally tell them that it is just a matter of time before they leak. Even with properly installed skylights, condensation and overflowing of small drip troughs is likely during cold weather. Roofing and flashings also wear out, over time, and small openings can lead to big problems.

Sometimes this leakage can be very minor, with a few drops of water seen on the floor below the skylights on cold days, and other times it is a more serious case like in your home. If the skylights are lined with moisture-resistant material and the flooring below these areas have a waterproof surface, few problems may occur.

However, if the flooring is hardwood or laminate and the skylights lined with standard drywall, damage can be more extensive. While your efforts may seem drastic, they may be the most sensible to prevent a recurring problem.

As far as looking for a shortcut, you are out of luck. Do it once, do it right.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.